1. Field of the Invention
The present invention relates to a semiconducting ceramic, and, more particularly, to a semiconducting ceramic comprising barium titanate. The present invention also relates to a semiconducting ceramic electronic element formed from the semiconducting ceramic.
2. Related Art
Conventionally, electronic elements for controlling temperature, limiting current, generating heat at constant temperature, and like applications have employed a semiconducting ceramic having a positive-temperature-coefficient characteristic (hereinafter referred to as a PTC characteristic); i.e., wherein electric resistance thereof drastically increases at a temperature higher than Curie temperature. With regard to such a semiconducting ceramic, barium titanate ceramics have widely been used.
In recent years, there has arisen demand for a semiconducting ceramic electronic element for the above applications which has a high withstand voltage (i.e., high dielectric strength) and thus can be used at high voltage. Particularly, a semiconducting ceramic electronic element employed in a overcurrent-protecting element for electric circuits must have high withstand voltage.
The dielectric strength of a semiconducting ceramic is generally known to increase when the grain size thereof decreases. Therefore, studies have been conducted for forming micrograms of a semiconducting ceramic in order to increase the withstand voltage. For example, Japanese Patent Application Laid-Open (kokai) No. 4-26101 discloses that incorporation of Dy2O3 and Sb2O3 into a semiconducting ceramic formed of SrTiO3 and BaTiO3 containing TiO2, SiO2, Al2O3 and MnO2 provides a semiconducting ceramic having high withstand voltage and low specific resistance. Although the specific resistance is as low as 50 xcexa9xc2x7cm, the dielectric strength is about 200 V/mm, which is still unsatisfactory. In addition, the grain size is disadvantageously as large as 6-15 xcexcm.
Japanese Patent Publication (kokoku) No. 60-25004 discloses that a semiconducting ceramic having a grain size of 1-5 xcexcm and a maximum dielectric strength of 500 V/mm is obtained by crushing and mixing barium titanate and Sb oxide serving as a semiconducting agent; calcining under controlled conditions; compacting under controlled conditions; and firing the compact at 1350xc2x0 C. However, the dielectric strength is still insufficient for fulfilling the recent demand for high withstand voltage, and higher dielectric strength is required.
Meanwhile, in order to enhance dielectric strength, attempts have been made to reduce the grain size of a semiconducting ceramic to 1.0 xcexcm or less. However, when the average grain size of the semiconducting ceramic decreases, the specific resistance of the ceramic at room temperature disadvantageously exhibits an increase over the course of time (hereinafter referred to as xe2x80x9ctime-course increase.xe2x80x9d)
There is a relationship between the ratio represented by BaCO3/BaO and time-course change of specific resistance at room temperature. When barium titanate is synthesized from barium carbonate and titanium oxide, unreacted barium carbonate generally remains on the surfaces of barium titanate particles in a trace amount. Unreacted barium carbonate provides barium oxide during firing. Although the barium oxide remains in ceramic grains immediately after firing, when the ceramic is allowed to stand in air the barium oxide reacts with the carbon dioxide in the air to thereby segregate in grain boundaries as a barium carbonate phase having high electric resistance. The present inventors have found that the barium carbonate phase formed in the grain boundaries causes an increase of specific resistance at room temperature, and the increase can be prevented by controlling the relative spectral intensity ratio represented by BaCO3/BaO of the barium titanate-based semiconducting ceramic. The present invention has been accomplished based on this finding.
An object of the present invention is to provide a semiconducting ceramic which has a dielectric strength of 800 V/mm or more and a specific resistance at room temperature of 100 xcexa9xc2x7cm or less, the specific resistance at room temperature undergoing substantially no time-course change.
Another object of the present invention is to provide a semiconducting ceramic electronic element produced from the same.
Accordingly, in a first aspect of the present invention, there is provided a semiconducting ceramic formed of a sintered semiconducting material containing barium titanate, which ceramic has an average grain size of about 1.0 xcexcm or less and a relative spectral intensity ratio represented by BaCO3/BaO of 0.5 or less as determined by XPS at a surface of the ceramic.
The semiconducting ceramic according to the first aspect of the present invention possesses a dielectric strength of 800 V/mm or more and a specific resistance at room temperature of 100 xcexa9xc2x7cm or less, the specific resistance at room temperature undergoing substantially no time-course change.
In a second aspect of the present invention, there is provided a semiconducting ceramic electronic element in which an electrode is formed on each major surface of the semiconducting ceramic as described in the first aspect of the present invention.
The specific resistance of the semiconducting ceramic electronic element according to the second aspect of the present invention exhibits no time-course change at room temperature. In addition, dielectric strength is also enhanced due to barium titanate having an average grain size of 1.0 xcexcm or less. Furthermore, the thickness of the semiconducting ceramic electronic element can be reduced to as little as 0.7-1.0 mm, whereas the thickness of a conventional semiconducting ceramic electronic element is approximately 2 mm.
As described above, the present invention provides a semiconducting ceramic formed of a sintered semiconducting material containing barium titanate, wherein the average grain size of the semiconducting ceramic is about 1.0 xcexcm or less and relative spectral intensity ratio represented by BaCO3/BaO as measured by XPS at a surface of the ceramic is 0.5 or less.
Barium titanate used in the semiconducting ceramic according to the present invention is not limited only to BaTiO3; there may also be used as the barium titanate of the present invention a barium titanate-based species in which the Ba is partially substituted by Sr, Ca, Pb, Y, a rare earth element, etc., or the Ti is partially substituted by Sn, Zr, Nb, W, Sb, etc.
MnO2, SiO2, TiO2, Al2O3, etc. may be added in an appropriate amount to a semiconducting material containing barium titanate for producing the semiconducting ceramic according to the present invention.
The average grain size of the semiconducting ceramic according to the present invention is limited to about 1.0 xcexcm or less, although the ceramic may contain ceramic grains having a grain size in excess of about 1.0 xcexcm.
The relative spectral intensity ratio represented by BaCO3/BaO at the surface of the semiconducting ceramic must be 0.50 or less. As used herein, the term xe2x80x9csurfacexe2x80x9d refers to a portion of the semiconducting ceramic that is in contact with air. Thus, a cut section is also included.
In the present invention, the expression xe2x80x9cspecific resistance at room temperature undergoing substantially no time-course changexe2x80x9d refers to a case in which the ratio of specific resistance at room temperature 1000 hours after the termination of firing to that immediately after firing is 1.05 or less.